Suspension devices for vehicle seats are known in particular for damping a height excursion of the vehicle seat when travelling over uneven road surfaces, such as potholes, EP 1 188 608 A1 discloses an active damping system for sprung vehicle seats in which, in addition to a mechanical flexible connection device, a pneumatic off-load device and a hydraulic actuator are arranged between a seat part and a lower part which is connected to the vehicle. Both the pneumatic off-load device and the hydraulic actuator are controlled by a control device as a function of a seat error signal, which is generated for example by a jerky height adjustment of the seat part.
Such hydraulic actuators require a connection to the on-board power supply of the vehicle, in particular a utility vehicle, such as a tractor. Such a connection means that the action of the active damping of the vehicle seat is dependent on the function of the on-board power supply and in particular the parameters thereof, so that the parameters of the damping system have to be matched to the parameters of the on-board power supply and thus the system has to be adapted to the respective utility vehicle. Moreover, such suspension devices are of simple design in so far as the hydraulic actuator, upon receipt of a position error signal in relation to the vehicle seat, which is subjected to the weight of a driver, is supplied with air by means of a compressor and as a result an increased air volume is created for sufficient damping of the vehicle seat along their height adjustment, such suspension devices do not have any comfort region which extends over a relatively long section of the height adjustment travel and thus represents uniform suspension within this height adjustment range with the same return forces.
Also known are air springs which have a linear force/travel air spring characteristic, the incline in which differs as a function of the configuration of the air spring and an applied additional volume. In such air springs, use is usually made of additional volumes which are kept constant and which are associated with the extending and retracting air spring as the actual air volume of the air spring.
Such air volumes which are kept constant mean that, when the air spring is set with a force/travel air spring characteristic having a small incline, it is not possible for the seat part to swing back into a central position of the force/travel air spring characteristic since friction forces within the suspension device as a whole are greater than a return force within the force/travel air spring characteristic.
On the other hand, if the suspension device is designed such that the return force is greater—that is to say a greater incline of the air spring characteristic is set—a relatively hard damping is achieved both in the central travel region and in the end-of-travel regions of the air spring.
Moreover, when using an air spring in an air spring characteristic range with a small incline (the so-called comfort range), which requires the switch-on of a large additional volume, it is probable that the end stops in end-of-travel regions of the air spring will be reached in the event of considerable extension and retraction movements due to considerable unevenness, as a result of which a reduced seating comfort is obtained for the user of the vehicle seat. In this case of air springs for vehicle seats, the setting of a comfort range is usually desired on account of better transmission values and the resulting higher level of comfort.
Such a comfort range of the air spring characteristic has to date been set by the manufacturer such that it can be used universally for different roads and ground with different levels of unevenness. Since tractors in particular, which travel both on tarred roads and on dirt tracks or even on fields, have very different requirements in terms of optimizing the suspension device or a suspension system for the tractor vehicle seat, a universal comfort range that has been set from the start can be only a compromise for effective suspension of the vehicle seat on different types of ground.